The present invention relates to a system for controlling an electro-magnetic clutch for automobiles, and more particularly to a system for an electro-magnetic powder clutch which may decrease the shock at the change of the transmission ratio.
The electro-magnetic clutch as disclosed in the U.S. patent application Ser. No. 195,897 comprises an annular drive member secured to the crankshaft of the engine, a magnetizing coil provided in the drive member, a driven member secured to the input shaft of the transmission with leaving a gap to the drive member, and powder provided in the clutch. The powder is made of magnetic material such as chromium steels. When the accelerator pedal is depressed, electric current flows through the magnetizing coil to magnetize the drive member. The powder is aggregated in the gap by the magnetic flux, so that the driven member is coupled to the drive member through the powder. Thus, the output of the engine may be transmitted to the transmission.
The clutch current in the magnetizing coil, that is the clutch torque is progressively increased according to the engine speed, while the clutch slips between the drive member and the drive member. Thus, the vehicle may be smoothly and easily started by depressing the accelerator pedal without operating the clutch pedal.
FIG. 1 shows the variation of the clutch torque during the speed changing. When the shift lever for changing gears is shifted to the neutral position, a shift lever switch is opened, so that the clutch torque goes to zero. When the shift lever is shifted to a speed changing position, the shift lever switch is closed. Thus, the clutch current flows the magnetizing coil, so that the clutch torque increases 1/3 to 2/3 of the rated clutch torque as shown by A. Then, the clutch torque increases with the increase of the engine speed as curve B. At the end of the clutch engaging period T.sub.2, the clutch torque steeply increases as C. By the initial rise A of the clutch torque, the driver of the vehicle feels the engagement of the clutch and then accelerates the engine.
However, since the clutch torque increases with the engine speed as shown by B, the clutch torque rapidly increases if the engine speed is greatly increased as the transmission is shifted down. Because of a rapid increase of the clutch torque, an undesirable shock occurs. If the partial engagement period of the clutch is elongated, such a shock may be reduced. However, a long partial engaging condition of the clutch reduces the engine braking effect.